Most of substrates for flexible printed circuit boards, etc. have been so far manufactured by bonding a metallic foil and an aromatic polyimide film with an adhesive such as epoxy resin, polyurethane resin, etc. However, the flexible printed circuit boards manufactured with such an adhesive have problems such as adhesive peeling due to successive thermal compression bonding hysteresis or due to exposure to elevated temperatures in the soldering step, or smear generation in the drilling step owing to an adhesive, and further have such drawbacks as curling, twisting, warping, etc. of the substrates after cooling, causing trouble particularly to form a fine pattern.
These problems owe their origin to differences in coefficient of linear thermal expansion between a conductor and an insulating material, and thus it has been proposed to improve the heat resistance of the adhesive. Basically, omission of the adhesive layer can solve not only the problems due to the adhesive layer, but can also save the labor of bonding the adhesive to the polyimide film.
From such a viewpoint, a metal laminate has been formed in some cases by directly coating a metallic conductor with polyamic acid as a polyimide precursor copolymer, followed by heating to effect polyimidization. It is known that the metal laminate thus obtained has a poor dimensional stability and suffers from curling.
To overcome these drawbacks, JP-B-5-22399, JP-B-6-93537, JP-B-7-39161, etc. disclose metal laminates with distinguished dimensional stability, adhesiveness, flatness after etching, reduction in curling, etc., manufactured by forming a plurality layers of a polyimide resin layer having low thermal expansion and other polyimide resin layers on a conductor, where two or three kinds of polyimide precursor copolymers must be used, the individual copolymer solutions must be applied one by one to the conductor to form an insulation multilayer, and a ratio in thickness of the resulting individual polyimide layers must be specified, inevitably complicating the manufacture of the metal laminate thereby.
The present applicant has already proposed a metal laminate where one kind of polyimide copolymer layer is directly laminated to a metallic conductor (WO 01/29136). The polyimide copolymer used therein is a copolymer of isopropylidenebis(4-phenyleneoxy-4-phthalic acid) dianhydride and 6-amino-2-(p-aminophenyl)benzimidazole. The polyimide copolymer obtained by polycondensation of these monomer components has a high adhesive strength by itself and can give a metal laminate with a satisfactory peel strength even if laminated directly to a metallic foil without interposing an adhesive layer therebetween, and also has a good solder heat resistance, but originally the polyimide copolymer has not been intended to lower the coefficient of linear thermal expansion or percent heat shrinkage or improvement of curling resistance.